Convert CONFIG_ENV_IS_IN_NVRAM to Kconfig

This converts the following to Kconfig:
   CONFIG_ENV_IS_IN_NVRAM

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/common/Kconfig b/common/Kconfig
index 29e25fe..2f86e7c 100644
--- a/common/Kconfig
+++ b/common/Kconfig
@@ -340,6 +340,22 @@
 	  Currently, CONFIG_ENV_OFFSET_REDUND is not supported when
 	  using CONFIG_ENV_OFFSET_OOB.
 
+config ENV_IS_IN_NVRAM
+	bool "Environment in a non-volatile RAM"
+	depends on !CHAIN_OF_TRUST
+	help
+	  Define this if you have some non-volatile memory device
+	  (NVRAM, battery buffered SRAM) which you want to use for the
+	  environment.
+
+	  - CONFIG_ENV_ADDR:
+	  - CONFIG_ENV_SIZE:
+
+	  These two #defines are used to determine the memory area you
+	  want to use for environment. It is assumed that this memory
+	  can just be read and written to, without any special
+	  provision.
+
 config ENV_IS_IN_UBI
 	bool "Environment in a UBI volume"
 	depends on !CHAIN_OF_TRUST